Lambdas with TFunctionRef<T>

TFunctionRef<T> is an Unreal-friendly way of letting you define and use
Lambda functions. If you are unfamiliar with Lambda functions in C++, you
should research what they are first before trying to use the following.

TFunctionRef<int32(constFString&str)>Thing

Imagine we have a function that takes a list of items as input, and it performs
some actions on each one, and then passes them on to another internal object.
We might want to let the user specify a custom initializer function that is
called on each item before it is handed off.

We can do this by adding a TFunctionRef<T> parameter to our function, as
shown below.